1. Field of the Invention
The invention relates to a process for preparing 4-methyl-2-cyclohexylphenol from p-cresol and cyclohexanol or cyclohexene.
4-Methyl-2-cyclohexylphenol is used for preparing stabilizers for thermoplastics and elastomers (DE-A-3,021,726).
2. Description of the Related Art
4-Methyl-2-cyclohexylphenol can be prepared in "moderate" yield (no further information given) by reacting p-cresol with cyclohexanol in 72% strength sulphuric acid at 60.degree. C. (J. Prakt. Chem. N.F. 159 (1941), 155, especially 164).
It has also become known to catalyse this reaction with aluminum chloride or phosphoric acid (Zh. Organ. Khim. 1 (1965), 510 of the English translation).
In both cases the reaction is carried out at 90.degree.-100.degree. C.; in the case of AlCl.sub.3, an equimolar amount of catalyst but 5 times the molar amount of p-cresol, based on the cyclohexanol, is used, and the catalyst is added to the mixture of the reactants. In the case of phosphoric acid, it is introduced first in a 3 times molar excess over the p-cresol together with the latter, and an amount of cyclohexanol which is equimolar to the p-cresol is then added. The yields are reported as 61% (AlCl.sub.3) and 78% (H.sub.3 PO.sub.4).
Zh. Prikl. Khim. 41 (2) (1968), 381 of the English translation, describes the reaction of p-cresol with cyclohexene in the presence of acidic ion exchangers. This reaction always produces, even with an excess of p-cresol, a substantial proportion of cyclohexyldisubstituted p-cresol. In addition, considerable amounts of cyclohexyl p-tolyl ether are formed. If the alkylation is carried out with cyclohexanol under such conditions, the predominant reaction product is cyclohexyl p-tolyl ether.
Furthermore, U.S. Pat. No. 1,917,823 discloses reacting phenol and o-cresol with cyclohexene or cyclohexanol in the presence of bleaching earths. This reaction is in every case carried out under high temperatures and superatmospheric pressure. According to this U.S. Patent, the alkylation in the ortho position is not the final substitution position; on the contrary, the ortho product is converted into the para product by the higher reaction temperatures; if o-cyclohexylphenol is added at the start, even the further formation of this product is suppressed.
In DE-A-2,437,322, the catalysts used for the alkylation by means of alkenes are molecular sieves. However, in this reaction the phenolic reactant used is m-cresol, which in a first stage is easily converted into a mixture of the various isomeric alkyl-m-cresols which, in a second step, produce the thermodynamically stable 5-alkyl-3-methylphenol by isomerization. The working examples of this DE-A do not report any yields. Instead, what is indicated in the first reaction step are always differently composed mixtures of the theoretically possible isomers; this applies not only to the alkylation products predominantly prepared with propene but also to some reaction products prepared with 1-butene, cyclohexene and styrene.
It is clear that earlier catalysts, such as H.sub.2 SO.sub.4, AlCl.sub.3 and H.sub.3 PO.sub.4, which are frequently used in large amounts, present substantial disposal problems. Moreover, it is plain that insoluble catalysts, such as ion exchangers, bleaching earths and molecular sieves, lead in some instances to different products or are successfully usable only on specific substrates.